The hydroformylation reaction, also known as the Oxo Reaction or Oxo Process, consists in reacting a synthesis gas made up of a mixture of carbon monoxide and hydrogen and at least one CnH2n olefin so as to obtain an aldehyde product containing n+1 carbon atoms. The oxonation reaction is generally catalyzed with carbonyls of transition metals such as cobalt or rhodium, optionally in presence of a ligand to stabilize the carbonyl complex of the metal. The aldehyde may then be catalytically hydrogenated to the corresponding alcohol, but optionally some or most of the aldehyde may be separated off for use as such or for conversion into higher molecular weight alcohols, as described below, or into carboxylic acids, optionally also those having the higher molecular weight. Such a process sequence of hydroformylation followed by hydrogenation may lead to the production of n-butanol, isobutanol, mixtures of C4 alcohols, n-pentanol, 2-methyl-butanol, 3-methyl butanol, mixtures of at least two C5 alcohols, or higher alcohols such as isoheptyl alcohol, iso-octyl alcohol, isononyl alcohol, isodecyl alcohol, isoundecyl alcohol, or isotridecyl alcohol. The Oxo Process, as well as its hydrogenation step, is described in detail in patents too numerous to recite. It is commercially highly important, producing products that find uses in plastics, soaps, lubricants, and other products.
Alternative processes for producing alcohols may start with the hydroformylation of lower carbon number olefins, such as ethylene, propylene and butenes to the corresponding aldehyde or aldehyde mixtures containing one more carbon number than the starting olefin or olefins, as already mentioned above. These aldehydes, or mixtures thereof, are then subjected to aldolisation to produce condensation products, typically higher aldehydes containing at least one extra carbon-carbon double bond, often referred to as enals. These enals or enal mixtures may be hydrogenated to the corresponding saturated aldehydes or aldehyde mixtures, or directly to the corresponding alcohols or alcohol mixtures. Examples of products produced by such processes are 2-methyl pentanol, 2-ethyl hexanol, 2,4-dimethyl heptanol and 2-propyl heptanol, but other alcohols and alcohol mixtures produced in this way are also known.
In conventional commercial processes, the desired alcohol fraction after the first hydrogenation section as part of the various processes as described above typically contains about 0.05 to about 0.5 weight % carbonyl-containing compounds, particularly aldehydes. The presence of such an amount of carbonyl-containing compounds generally leads to poor color and undesirable odors in derivatives, such as plasticizers, produced from the alcohols. The alcohol fraction is therefore generally subjected to hydrofinishing to reduce its carbonyl level. This is a problem because of the requirement of numerous additional reaction steps along with attendant apparatus. An alternative to hydrofinishing is to treat the alcohol with NaBH4. This is a less complex process step, but the sensitivity of NaBH4 to water introduces a safety risk that must be controlled. It would be highly beneficial if a process/system could be devised to eliminate these steps of reducing residual aldehyde, or to make the elimination of traces of aldehydes a more simple process, system or method.
The present inventors have surprisingly discovered that the formation of acetals and/or unsaturated ethers by the reaction of alcohols and aldehydes may be exploited to produce high purity alcohols and avoid some or all of the requirements of hydrofinishing or other carbonyl reduction steps.
The formation of acetals is the subject of numerous patents. Acetals are important as fragrances, pharmaceuticals, flavorings, and as oxygenated additives to reduce particulate and NOx emissions on the combustion of fuels.
In U.S. Pat. No. 2,519,540, acetaldehyde and ethanol are converted to diethyl acetal in the presence of an acidic catalyst, such as sulfuric acid or phosphoric acid, and inert diluent immiscible with water, such as kerosene, n-hexane, carbon tetrachloride, and the like. The acetal is recovered from the diluent phase by distillation.
U.S. Pat. No. 2,668,862 is directed to an improved process for producing and recovering higher molecular weight acetals. The alcohol and aldehyde are reacted in the presence of a catalyst such as (preferably) anhydrous hydrogen chloride at room temperature. Water formed during the reaction is absorbed by the addition to the reaction mixture of a dehydrating salt such as sodium sulfate or calcium chloride. The acetal product is isolated by adding an aqueous alcohol to the reaction product, causing the acetal to separate out as a bottom layer from which is can be drawn off after settling.
U.S. Pat. No. 6,015,875 produces acetals by the reaction of aldehydes and alcohols, separately added to a reaction distillation column, in the presence of a catalyst. The process includes the concurrent fractional distillation of the reaction mixture to separate the reaction products, acetal as overheads and water as bottoms.
U.S. Pat. No. 6,214,172 teaches preparation of methylglyoxal dimethyl acetal by reacting 2-oxopropanal with methanol in the presence of an acidic ion exchanger. The product acetal is obtained by azeotropic distillation with water, with the mixture separating at the top of the distillation column into an aqueous phase and acetal phase.
U.S. Pat. No. 6,518,464 relates to a process for preparing unsaturated acetals by reacting aliphatic olefins with allyl alcohols in a reaction column, where the reactants are only partially reacted in the reaction column. Vapor comprising alcohol and aldehyde are taken overhead from a reaction column and, after separation from water, are returned to the top of the reaction column as reflux. The acetal is removed as bottoms and then concentrated in two stages.
Silva and Rodrigues disclose the production of acetal catalyzed by an acid resin using simulated a moving-bed reactor. Acetaldehyde conversion is reported to be about 98%. See Silva et al., Novel Process for Diethylacetal Synthesis, AIChE Journal, Vol. 51, No. 10, pp. 2752-2768 (October 2005). A batch-scale comparison of the production of acetal by an acid resin and a zeolite catalyst was also reported by Gandi, Silva, and Rodrigues in Ind. Eng. Chem. Res. 2005, 44, 7287-7297.
However, as far as the present inventors are aware, the prior art has not purified the alcohol product of the Oxo Process by using the reaction of aldehydes and alcohols to make acetals and/or unsaturated ethers.